Chill Requirements of Fruit Trees

Many fruit and nut trees require a certain amount of winter chill in order to fruit productively. As the temperatures cool down the buds on the trees go into a period of dormancy. In this dormant state they are able to withstand cold temperatures without damage. The trees then do not come out of this dormant state until after they have received the required amount of winter chill.

If the buds do not receive sufficient chilling temperatures during winter to completely release dormancy, trees may develop physiological symptoms such as:

delayed and extended bloom;

delayed foliation;

reduced fruit set; and

reduced fruit quality.

The chill requirement is often expressed in chill hours, which can be calculated in different ways, all of which essentially involve adding up the total amount of time in a winter spent at certain temperatures.

A list of fruit and nut varieties and their chill requirements is here.

Common Chill Accumulation Models

There are a number of different models used to calculate the chill accumulated at a site. Ideally the same model should be used for the site calculation as that used when determining the requirement of a specific fruit variety. However most literature that provides the chill requirements to fruit and nut varieties for the home gardener do not bother to state which model was used.

7°C and Under Model

This is the simplest model. Each hour under 7°C is counted. That is it.

0-7°C Model

A slightly more complex model is one where temperatures under zero degrees are ignored. Only hours between 0 and 7°C are counted.

Utah Model

Scientists noticed that under controlled condition's temperatures of 6°C attributed most towards the completion of tree rest than any other temperature. So, one hour at 6°C was given the value of 1 chill unit (CU).

The same scientists also found that lower and higher temperatures could counter-act those effects. Scientists at Utah State University in Logan, developed the following model to help calculate chill units received.